For the same reason they make cars and tires that will go in excess of 100 mph... they both can, but for a variety of reasons, it's not a good idea to most of the time.
The reason for cars should be obvious (if not, try driving through a school zone or past a cop shop in excess of 100 mph and you'ff quickly find out what I mean).
The same thing with burners... go ahead and burn at 16X or whatever the media will support, just don't expect to be pleased with the results.
I don't know how many different types of media you have lying around, but it's a good idea, if you're going to use Imgburn, to set them all up, so you don't have to worry about it later. I usually set at half the max speed, and my burns so far are flawless.